Hepatitis C virus (HCV) causes a very high frequency of persistent infection, which leads to chronic hepatitis, cirrhosis and hepatocellular carcinoma. The mechanisms of viral pathogenesis and persistency are not clear. Our laboratory has obtained preliminary findings that the HCV core protein can interact with several members of tumor necrosis factor receptor (TNFR) family, including TNFR I, TNFR II, lymphotoxin-beta receptor (LTbetaR) and Fas. These receptors are important components of the cellular immune defense mechanism and are involved in cellular apoptosis. Thus, the interaction between the HCV core protein and these receptors can potentially disrupt cellular defense, accounting for viral persistent infection. Alternatively, these interactions could sensitize hepatocytes to TNFs, leading to hepatitis. These findings thus provide new avenues for studying HCV persistency and pathogenesis. In this project, four specific aims are planned: 1. To characterize the biochemical properties of these interactions. 2. To study the biological effects of these interactions, including the effects of HCV core on the cellular responses to TNFs and the signal transduction of these receptors. 3. To use primary hepatocytes derived from HCV(+) livers to examine the possible effects of cytokines or hormones on HCV replication. 4. To characterize the molecules other than TNFRs detected in the yeast two-hybrid system, which may interact with the HCV core protein. These projects are expected to reveal the potential role of the core protein in HCV persistency and pathogenesis, and may contribute to the establishment of an in vitro culture system for HCV. These two goals are the major theme of this Hepatitis C Cooperative Research Center. Our proposed projects represent new directions for the research in these areas. They also form the basis for Project II (HCV core transgenic mice) and Project IV (tumor necrosis factors receptors, lymphocyte and HCV therapy.)